Hidden damage (Purple Heart Valley). Rules for daylight bombing in WW2. When hit the shooter rolled D100 and the owner of the hit aircraft told them what they saw and noted down what had actually been done. Other than engines catching fire, or the plane exploding, the attacker didn't know how badly damaged a target was and if it was necessary to keep attacking. For example you would frequently see "bits fly off" but that could be anything from a few holes in the plane to spreading the tail gunner down the fuselage.